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Hundreds were injured in the bomb attacks in Sanaa

The bombing of two mosques in Sanaa, which killed close to 140 people and injured 350, represents another stage in Yemen's spiralling violence.

But the significance of the bombing, which was claimed by Islamic State (IS) militants, goes beyond the horrendous human loss. If IS indeed stands behind the attack, it would not only further complicate the situation in Yemen, but also have implications for the broader struggle against IS.

IS's need continuously to expand is a central feature in its strategy since its blitzkrieg in Iraq and Syria last year gave it control over vast territory and facilitated the announcement that it is a caliphate.

Viewing itself as the rightful leader of Muslims everywhere and as mandated by God to rule over all territories once under Muslim control and beyond, IS must spread.

Expansion is also a mobilisation tool for IS. It adds to the myth of Islamic State's inevitability and invincibility that its leaders are trying to promote.

By building its brand name, IS seeks to convince Muslims to accept its authority and pledge their allegiance to the self-styled caliph, Abu Bakr al-Baghdadi.

But IS miscalculated. Early victories, astonishing as they were, reflected the poor state and morale of its opponents in the Iraqi army and the lack of advanced armament by the Syrian rebels more than the prowess of its own forces.

When IS pursued the genocide of the Yazidis, threatened Irbil, the capital of the Kurdish autonomous region, and beheaded American hostages, it forced the United States to intervene.

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Thousands of Yazidis have been abducted or killed by Islamic State

The American intervention largely stopped IS's advance and, in some locations, even forced IS forces to retreat after suffering heavy casualties. Moreover, to its dismay, the American assault did little to make prominent jihadi scholars and rival groups (primarily al-Qaeda) change their minds and pledge allegiance to al-Baghdadi.

At the same time, the attacks IS suffered have hardly been devastating. The complex conditions in Iraq and Syria, as well as the American fear of getting bogged down in another ground war in the Middle East, led an intervention-weary President Barack Obama to adopt a very cautious strategy.

As a result, the US and the mostly symbolic coalition it formed did not make any effort to uproot IS from its main strongholds, and failed to convince its supporters that the caliphate project is doomed.

Loyal to its slogan "staying and expanding" but unable to continue its expansion in the main battlefront, Islamic State had to prove its viability and success elsewhere.

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IS attacks have prompted anti-terror protests in the Middle East

In November 2014, it announced that it accepted pledges of allegiance from jihadis in Libya, Algeria, the Sinai Peninsula, Yemen, and Saudi Arabia. As a symbol of its alleged authority, it also announced the establishment of wilayat (governorates) in these countries.

Since the beginning of the year, IS has also collected oaths of fealty from the former Afghan and Pakistani Taliban and from the Nigerian Boko Haram militant group, and declared the establishment of wilayats there.

Notwithstanding the fanfare surrounding these announcements, in reality IS's presence and power outside Iraq and Syria is much more limited.

The stronger groups who joined IS, Ansar Beit al-Maqdis in Sinai and Boko Haram in Nigeria, are in decline. In the case of the latter, weakness was probably the reason it joined IS, and it is unclear to what extent it accepts al-Baghdadi's authority.

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The Boko Haram pledge of allegiance to IS could be a sign of weakness

In Libya, IS is still a weak force, although it is taking advantage of the civil war in the country to slowly gain traction. The IS presence elsewhere is even more fleeting.

But as the experience of Iraq and Syria and IS's growing profile in Libya suggest, the group strives in unstable places, that are in large supply throughout the Middle East and Africa. This makes Friday's attack in Yemen particularly alarming.

'No US plan'

If IS was indeed responsible for the carnage - the US expressed doubts regarding the veracity of the claim - it would symbolise its actual entry to the Yemen arena.

The rise of the Shia Houthis did not reflect sectarianism initially, but the conflict has assumed a much more pronounced sectarian character since the Houthis forced President Abdrabbuh Mansour Hadi out.

This is particularly bad news because if Syria is any indication, such a conflict draws in radical Sunni Islamists and makes compromise extremely difficult to reach.

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There are fears that sectarian violence is on the increase in Yemen

The Sunni-Shia fault line already strengthened the position of al-Qaeda's Yemeni branch. But whereas AQAP still listens to the wishes of its tribal allies and acts with some restraint (it quickly denied responsibility for the mosques' bombing), the introduction of Islamic State would lead to further radicalisation of the conflict and rise in indiscriminate violence.

Beyond Yemen, the Sanaa bombings underscore how the American strategy against Islamic State is lacking. The US is not degrading IS fast enough to stop its expansion, and it does not appear to have a plan for confronting IS branches in Sinai, Libya, West Africa, and now Yemen.

In taking its time to address a quickly expanding threat, the US not only risks further turmoil in the Middle East and Africa, but also makes attacks by IS members in the West more likely.

Barak Mendelsohn is Associate Professor of Political Science at Haverford College and a Research Fellow at Harvard Kennedy School's Belfer Center for Science and International Affairs.